1. Field of the Invention
This invention relates generally to curing technology, in particular, this invention is a system and method for curing waterbased coatings.
2. Description of Related Art
An air atomizing spray gun is typically utilized to rapidly apply paints, industrial coatings and other finishing products to a wide variety of industrial, commercial, and consumer goods. Unfortunately, a profusion of transient, airborne particles and associated fumes, generally designated as overspray, are produced during the application process. To reduce the potentially serious health risks associated with the inhalation and bodily contact of the overspray, spray booths and other collection systems have been designed in accordance with a plethora of strict regulations. These regulations are set forth by the Occupational Safety and Health Administration (OSHA), the Environmental Protection Agency (EPA), the National Fire Protection Association (NFPA) and a myriad of other governmental regulatory agencies, to collect and effectively treat the discharged air and direct it away from the operators of the spray equipment and other adjacent ancillary personnel. Heretofore, high volume blowers have typically been utilized to draw uncontaminated, ambient air through the coating area, where the air mixes with the overspray, and to duct the air, now contaminated with coating particles and noxious gases, into a treatment area prior to discharge.
A dry filtration system, utilizing arrestor pads, has commonly been employed to remove overspray from the contaminated air stream. As the contaminated air stream passes through an arrestor pad, the larger coating particles impact against the surface of the pad and adhere thereto.
Solvent based coatings have commonly been utilized in finishing processes due to the fast drying characteristics of the solvents. As the solvents evaporate, the coating solids suspended therein flow together and form a continuous layer of dry solids. A major disadvantage of solvent based coatings is the explosion hazard created by the inherent flammability of the solvent and the associated solvent fumes which are released during the evaporation process. Additionally, the solvent fumes discharged to the atmosphere pose an environmental hazard due to the interaction of solvents with the ozone layer. Furthermore, solvent based coatings have the disadvantages of toxicity, intense odor, volatility, skin irritancy, carcinogenicity, high film shrinkage, loss of adhesion, variable cure speeds, and change in overall film properties upon application. As such, alternative coating processes utilizing dry powders, high solids and waterborne solids have been developed to avoid the disadvantages associated with solvent based coatings.
In a dry powder coating process, an electrostatic spray gun assembly having a positive polarity is utilized to apply dry powder solids to a product having a negative polarity. Due to the resultant mutual attraction of the positively charged paint particles and the negatively charged product, overspray is substantially reduced. After receiving the dry paint particles, the coated product is baked at a high temperature until the dry paint particles melt and flow about the product, thereby forming a continuous coating. Such systems require substantial investment for equipment and have limited use due in part to the required baking step.
High solids coating systems utilize a high viscosity paint emulsion having a high solids to solvent ratio. As a result, the paint emulsion is generally applied to a product with a high pressure spray nozzle which inherently produces a substantial amount of overspray. The coated product is subsequently cured in a separate drying area using a heat source such as an oven or heat lamps. As with the above-described powder coating systems, a high solids coating system requires a substantial investment for equipment and has limited use due to the required heating step.
In a waterborne solids wet system, the coating solids are suspended in a fluid having a relatively high water to solvent ratio. Although the equipment required for this type of coating system is generally less expensive and complex due to a lower curing temperature, the required drying times are generally much longer than with solvent or dry powder based coatings. Waterbased coatings are inexpensive and have the advantages of performance comparable to undiluted oligomers, excellent gloss, good chemical resistance, versatility in roller coating and screen printing, easier to clean, and reduced ecological problems. The main advantage of water based coatings is that they can be tailor-made to suit special applications. Water, when used as a diluent, shows a dramatic viscosity reducing effect. Variable viscosity can be manipulated effectively to suit various applications.
A major disadvantage of water based coatings is the need for water removal as a separate step prior to curing. Water removal is difficult because it has high latent heat of evaporation; hence at high temperature, a high energy input is required to facilitate drying, while at ambient temperature and/or high relative humidities drying is very slow. In industrial applications, longer drying/curing times decrease productivity and become quite costly to the manufacturing process.
As stated, currently available collection systems are generally designed to discharge large quantities of air to the outside environment. Unfortunately, this results in higher energy costs since additional energy must be expended to recondition the indoor building air. In addition, the residual pollutants in the discharged air are closely regulated by local and federal agencies, oftentimes requiring the procurement of a plurality of costly permits and/or the payment of large fines. These energy and regulatory requirements oftentimes add considerable cost to the price of a finished product.
Over the last decade, the use of high solvent based coatings has drastically decreased due to the ever increasing number of regulatory restrictions on the emission levels of contaminated air into the environment. As such, the popularity of dry powder, high solids, waterborne and other alternative coatings has increased tremendously. Due to the high investment cost and limitations of the dry powder and high solids coatings, waterborne coatings stand out as the best alternative for economical use. As stated above, one of the major disadvantages of a waterborne coating system is the requisite longer drying cycle which results in substantially increased production costs. In view of the disadvantages of solvent based coatings, radiation curable, waterbased coatings are becoming more popular.